1. Field of the Invention
This invention concerns gradation data processing apparatus suitable for use in digital copying equipment or the like in which an image on an original document is read photoelectrically, electric image signals obtained by the reading are converted into electric digital signals and the image is printed on a sheet or the like based on the thus converted electric digital signals.
2. Description of the Prior Art
In known digital copying equipment
example, where an original document is read at a resolution of 300.times.300 dot/inch.sup.2 (an image element with a minimum area to be read is referred to as a picture element), the density of the thus read picture element is classified into 16 steps or gradations (for example, the density is classified equally from the white level to the black level into 16 digital words of steps) thereby forming 16 gradation data composed, for example of 4 bits. An intermediate tone or halftone image processing such as the dither method, the density pattern method or the submatrix method is applied to the gradation data to prepare binary data and a copy of the original image is prepared in a recorded/non-recorded pattern depending on the binary data. In the copying equipment of this kind, since an importance is attached to the reproducibility of an original image with the halftone continuously varying from the white level to the black level such as of photograph and picture, the contour becomes blurred in the copy of the binary image such as of letter or symbol and the resolution is degraded. While on the other hand, in a digital copying equipment adapted to compare an amplitude of the read out signal corresponding to the density of the picture element of the original with a predetermined threshold value, form a binary data depending on the comparison and prepare a copy of the original document in a recorded/non-recorded pattern in accordance with the binary data, although the resolution of the binary image in the copy is increased, it is impossible to express the density of the halftone image resulting in such an image extremely difficult to see.
As a countermeasure for such a disadvantage, there has been a digital copying equipment adapted to change the binarizing treatment for the gradation data according to the case of preparing a copy of a halftone image such as of photograph and picture and the case of preparing a copy of a binary image such as of letter and symbol. In this equipment, an operator designates the binarizing processing by the former (binarization by means of halftone image processing) upon preparing a copy from an original document having a halftone image, while designates the binarizing processing by the latter (binarization by the comparison with a predetermined threshold value) upon preparing a copy of an original document having a binary image.
However, in ordinary cases, most original documents can not clearly be distinguished as the original of halftone image or the original of binary image, but halftone image and binary image are present mixed on the original image, which is called a free format image. Accordingly, in the case of preparing a copy of such a free format image original, the operator selects the binarizing treatment depending on whether the reproducibility in the halftone image or the resolution in the binary image is considered more important in the copy. In this case, degradation in the resolution of the binary image or the reproducibility in the halftone image inevitably results in the copy of the free format image original.
Further, in copying equipment of this kind, since the continuous change in the density of the halftone image in the original such as of photograph or picture can be given only as the density change classified into 16 steps, discontinuous stepwise change in the density inevitably results in the copied image.
For instance, if a photograph of a portrait is copied by such an equipment, an unnatural map pattern appears in the contour of the face making it difficult to see the copy. The map pattern results because the density change, which is continuous in the original document, is expressed stepwise and, accordingly, it becomes less conspicuous by using the gradation data with more gradation number such as 32 gradations or 64 gradations.
By the way, an original document is usually read by using an array of photoelectronic conversion elements such as CCD that converts the intensity of light into electric signals. That is, the original document is illuminated by a lamp and the reflected light is received by each of the photoelectronic elements to obtain electric signals indicating the density of each of the picture elements. However, since the photoelectronic conversion element array has different characteristics in each cell thereof and the illumination by the lamp per se is not uniform, electric signals obtained from the respective cells are different even upon reading an original document of identical density and the error may sometime reach about .+-.25% of the value between the black and white levels. The gradation data of a greater gradation number suffers greater effect of the error. A photoelectronic conversion element array with less error shows poor yield, whereas use of a photoelectronic conversion element array with large error for improving the yield will require accurate detection and strict compensation for the error. However, this generally requires very complicated procedures. Accordingly, gradation data of 16 gradations have generally been obtained so far from the photoelectronic element array of this kind.
As has been described above, although the quality of the reproduced image for the halftone image can be greatly improved by using gradation data composed of 64 gradations than the gradation data made up of 16 gradations and, in addition, those printers capable of sufficiently printing the image in the 64 gradations (for example, laser printer) have been put to practical use. Image processing has mostly been carried out with the gradation data of 16 gradations and gradation data of 64 gradations are seldom used actually. In view of the above, a demand exists for expanding the gradation data, for example, 16 gradations into data of 64 gradations.
Thus, it is a primary object of the present invention to provide a gradation data processing apparatus capable of improving the resolution for a binary image as much as possible without degrading the reproducibility of a halftone image.
Another object of this invention is to provide an apparatus capable of improving the resolution of a binary image represented by black or white such as letter or symbol and, particularly, an apparatus capable of improving the resolution of a binary image in a so-called free format image in which binary images and halftone images such as photograph and picture are present together.
A further object of this invention is to provide an apparatus for converting the gradation data of a small gradation number into the gradation data of a large gradation number capable of reproducing an image with smooth gradation.
A specific object of this invention is to provide an apparatus for converting gradation data in accordance with a first gradation classification (for example, 4 bit data prepared by dividing the density into 16 gradations: hereinafter referred to as the original gradation data) into gradation data in accordance with a second gradation classification, an order of which is higher than the first gradation classification (for instance, 6 bit data prepared by dividing the density into 64 gradations: hereinafter referred to as expanded gradation data), on the respective image data obtained from small regions in a 2-dimensional distribution image.
This invention provides a gradation data processing apparatus comprising means for extracting gradation data corresponding to a first small region of an original image in a x-y 2-dimensional distribution respectively and gradation data corresponding to a second small region adjacent with the first small region, means for detecting whether the deviation between the content of the gradation data corresponding to the first small region and the content of the gradation data corresponding to the second adjacent small region is within a predetermined range or not, and means for generating data indicating a maximum value if the deviation exceeds the upper value of the range, while generating data indicating a minimum value if the deviation goes below the lower value of the range.
With such a constitution, since the data indicating the maximum value is generated if the deviation between the content of the gradation data corresponding to the noted small region and the content of the gradation data corresponding to the small adjacent region, the noted small region exceeds the upper value of the range, and the data indicating the minimum value is generated if the deviation goes beyong the lower value of the range, even if the gradation data are binarized by the halftone image processing as described above, the resolution of the binary image is increased because the maximum value represents the data to be recorded while the minimum value represents the data not to be recorded.
This invention also provides a gradation data processing apparatus comprising means for extracting the gradation data corresponding to a small region of an image in x-y 2-dimensional distribution and the gradation data corresponding to the small region in adjacent with the noted small region, and means for conducting addition through a weighting factor to the extracted gradation data thereby obtaining gradation data made up of a greater number of bits per word than that of the original gradation data.
With such a constitution, since the original gradation data corresponding to the noted small region and the original gradation data corresponding to the adjacent small region are applied with the weighting factor to obtain expended gradation data, it is possible to reproduce an image with the smooth density change inherent to the halftone image such as photograph and picture and also possible to form expanded gradation data due to the higher gradation classification from the original gradation data of lower gradation classification.
In a preferred embodiment according to this invention, the deviation is defined as a value obtained by subtracting, from the doubled value for the content of the gradation data corresponding to the noted small region, the value for the content of the gradation data corresponding to the small region adjacent to the noted small region in the direction y, and the value for the content of the gradation data corresponding to the small region adjacent to the noted small region in the direction x. Output means issues the gradation data corresponding to the noted small region if the deviation is within a predetermined range, the data indicating the maximum value if the deviation exceeds the upper value of the range and the data indicating the minimum value if the deviation goes below the lower value of the range respectively.
Specifically, the deviation is defined as .delta. represented by the equation: EQU .delta.=2.times.D-(D.sub.L +Du) (1)
where D represents the content for the gradation data corresponded to the noted small region, Du represents the content of the gradation data corresponded to the small region adjacent to the noted small region in the y direction and D.sub.L represents the content of the gradation data corresponded to the small region adjacent to the noted small region in the x direction. The generating means generates the value D if the deviation .delta. is between a first threshold value T.sub.1 and a second threshold value T.sub.2 (T.sub.1 .gtoreq..delta..gtoreq.T.sub.2), generates the maximum value MAX (for example, the gradation 15 in 16 gradations) if the deviation .delta. exceeds the first threshold value T.sub.1 (.delta.&gt;T.sub.1) and the minimum value MIN (for example, the gradation 0 in the 16 gradations) if the deviation .delta. is lower than the second threshold value T.sub.2 (.delta.&lt;T.sub.2) respectively as the gradation data.
Referring more specifically, since the density change in a halftone image is continuous, the density difference between the adjacent small regions on the original is extremely small, whereas since the density change in a binary image is discontinuous, there is a portion where the density difference between the adjacent small regions is extremely large. This portion is referred to as a contour. By emphasizing the density difference at the contour, the discontinuity in the density change of the binary image can be made distinct to improve the resolution.
Accordingly, in this case, the contours of the binary image is stressed and the resolution can be improved by using the maximum value MAX if the deviation .delta. is greater than b=T.sub.1 and the minimum value MIN if the deviation .delta. is smaller than -b=T.sub.2 as the gradation data.
In another preferred embodiment, the weighting factor is effected by adding the doubled value for the content of the original gradation data (16 gradations) corresponding to a noted small region, the value for the content of the original gradation data (16 gradation) corresponding to a small region adjacent the noted small region in the y direction and the value for the content of the original gradation data (16 gradations) corresponding to a small region adjacent to the noted small region in the x direction. That is, expanded gradation data (64 gradations) are defined as D' represented by the following equation: EQU D'=2.times.D+(D.sub.L +Du) (2)
where D represents the content of the original gradation data corresponding to the noted small region, Du represents the content of the original gradation data corresponding to the small region adjacent to the noted small region in the y direction and D.sub.L represents the content of the gradation data corresponding to the small region adjacent to the noted small region in the x direction. In this embodiment, since the expanded gradation data are determined while considering the density change of the image in the x and y directions, a reproduction of a smooth density change inherent to the halftone image can be realized by the expanded gradation data. That is, expanded gradation data of 64 gradations, which is capable of reproducing an image with excellent quality, can be formed with ease from the original gradation data of 16 gradations which are generally used frequently and can be prepared comparatively readily.
By the way, it is desired that the binary image (such as a letter) can clearly be distinguished on the boundary between the portion with the black and the portion without the black. However, since the processing represented by the formula (2) blurs the boundary portion as natural as possible, resolution for the binary image is degraded.
Accordingly, the edge or contours of the binary image can be emphasized in copying by setting the maximum value of the expanded gradation data with no expansion processing according to the equation (2) if the value .delta. of the equation (1) represented by: 2D-(D.sub.L +Du) is greater than b, while setting the minimum value of the expended gradation data with no expansion processing according to the equation (2) if the value .delta. is smaller than -b.
In view of the above, in the preferred embodiment, the edge of a binary image (such as a letter) is emphasized and the resolution is improved by setting the maximum value that is, value 63 in the gradation data, of 64 gradations if .delta.&lt;T.sub.1, where T.sub.1 is the first threshold value, as the expanded gradation data while setting the minimum value that is, value 0 in the gradation data of 64 gradations if .delta.&gt;T.sub.2, where T.sub.2 is the second threshold value, as the expended gradation data.